Den Haag on loan for the 2003–04 season. He made his debut for the club, starting the whole game, in a 1–1 draw against FC Volendam on 12 September 2003. Van Der Gun scored his first goal for ADO Den Haag, in a 4–1 loss against Feyenoord on 7 December 2003. Once again, he played twice against his parent club, Ajax, on two occasions on 9 November 2003 and 1 February 2004. Van Der Gun then scored three goals in three matches between 15 February 2004 and 29 February 2004 against Feyenoord, Roda JC and FC Volendam. He became a first team regular at ADO Den Haag and went on to make twenty–six appearances and scoring six times in all competitions. Once again, Van Der Gun was loaned out to ADO Den Haag for the 2004–05 season. He made his second debut for the club, starting the whole game, in a 1–0 loss against FC Den Bosch in the opening game of the season. Two weeks later on 29 August 2004, Van der Gun scored his first goal in his second spell at ADO Den Haag, in a 3–1 win against RKC Waalwijk. This was followed up by scoring against his parent club, Ajax, drawing 3–3. He then scored two goals in two matches between 31 October 2004 and 12 November 2004 against FC Groningen and RBC Roosendaal. Van Der Gun later scored two more goals later in the 2004–05 season, coming against Vitesse and Feyenoord. Once again becoming a first team regular at the club, he went on to make thirty–three appearances and scoring six times in all competitions. It was announced on 31 March 2005 that Ajax would not be renewing Van Der Gun's contract at the end of the 2004–05 season. Borussia Dortmund On 22 August 2005, Van Der Gun was signed by Borussia Dortmund on a one–year contract after impressing the club on trial and turning down a permanent move to join ADO Den Haag. His first game for Dortmund was for the reserve team against FC Gütersloh 2000 on 24 August 2005 and he was on the wings with David Odonkor, Salvatore Gambino and Delron Buckley. Van Der Gun made his Bundesliga debut for the club on match day three against MSV Duisburg on 28 August 2005, starting a match before former Dortmund coach Bert van Marwijk replaced him in the 62nd Minute for Euzebiusz